Drop unmaintained fastapi-xml dependency

[julienschuermans]

The fastapi-xml dependency is problematic:

• it does not support fastapi>=0.128.0, see fastapi-xml fails with fastapi==0.128.0 cercide/fastapi-xml#19,
• it has not been updated since October 2024

This means shapeshifter-uftp cannot update its fastapi version, which is needed to benefit from downstream security fixes in e.g. starlette>=1.0.1 (CVE-2026-48710)

• (fastapi <0.128 pins starlette <0.51)